Field of the Invention
The field of the invention is ladder polymers.
Description of Related Art
Renewable energy and materials is a rapidly growing field, the development of which is in higher demand than ever. One major branch of renewable energy is organic electronics and semiconducting materials. Organic semiconductors have several advantages over their silicon-based counterparts including renewability, their ability to be solution-processed into lightweight and flexible films, and their ability to have their properties easily tuned through chemical synthesis.
Important progress has been made towards making organic semiconductor technology ubiquitous in everyday uses. Technologies such as organic photovoltaics (OPVs) and organic batteries may provide a practical path to achieve low-cost, renewable energy harvesting and storage. Plastic polymeric power generation and storage sources offer intriguing opportunities for both portable solar cells and batteries, as such materials are potentially flexible, lightweight, and easy to fabricate through low-cost processing techniques. Further, organic molecules may offer malleable properties that are easy to tune through chemical synthesis.
Functionalized fullerenes, such as phenyl-C61-butryic acid methyl ester (PCBM), have been the most commonly used n-type electron deficient material in the active layer of OPVs. Although alternative n-type materials have been investigated, such alternative materials have not been able to supplant PCBM as the n-type material to blend with p-type conjugated polymers for OPVs. One of the drawbacks associated with PCBM is its weak absorbance in the visible and near-infrared regions of the solar spectrum. Another problem with using PCBM in heterojunction OPVs is the difficulty associated with controlling the morphology of the resulting films. In heterojunction-based OPVs, it is desirable to have interpenetrating regions of the polymer and PCBM for an ordered heterojunction active layer in an OPV device. As PCBM is a small molecule, blending the two materials and spin-casting the materials onto the desired substrate may result in a bulk-heterojunction (BHJ) OPV with “islands” of PCBM (and/or “islands” of p-type material). These “islands” are dead zones for charge-separated holes and electrons as they will not be able to reach the electrodes at either end of the solar cell. Thus, there is a need to create an alternative n-type material to PCBM.
In addition to creating a PCBM alternative, it is desirable to design materials with low band gaps. Low band gap materials based on fused, aromatic organic compounds are important to the development of organic photovoltaics (OPVs) as they can absorb the longer wavelengths of the solar spectrum that smaller aromatic units cannot (such as PCBM). Band gaps can be reduced by increasing the planarity of the conjugated backbone by minimizing various steric interactions between aromatic units. Steric interactions tend to cause backbone twisting that results in a pi-electron conjugation that is only effective over shorter, segmented distances. This distance is affected by the structure of different compounds and inherently varies between different materials. As the conjugation length is increased, the band gap will decrease continually. However, conjugated organic materials will reach a point at which additional conjugated units will no longer result in significant band gap decrease. In this way, increasing conjugation length quickly approaches an asymptotic value for the band gap.
One approach to synthesize highly planar, low band gap materials is by using polymeric systems that are fused through the majority of, if not the entirety of, a conjugated polymer (CP). A CP that is comprised entirely of fused rings is called a ladder polymer. Due to the reasons mentioned above, ladder polymers generally have very low band gaps. Further, an additional benefit of an extended fused ring system is better charge transport due to pi-stacking between adjacent polymer chains. One challenge associated with ladder polymers is poor solubility in organic solvents to remain solution processable.